1. Field of the Invention
The present invention pertains to satellite positioning systems, and more specifically to a spoofing detection system for a satellite positioning system.
2. Description of the Related Art
Satellite positioning systems, sometimes referred to as global positioning systems or GPS, are now well-known in the art. Such systems, for example, NAVSTAR-GPS are rapidly being used for determination of the geocentric position of mobile units, such as water and land vehicles, aircraft and survey equipment to name a few. Herein, such satellite positioning systems are referred to as simply GPS systems, and include among others, the aforesaid NAVSTAR and GLONASS GPS systems.
In aircraft, GPS systems are being utilized for navigation, flight control, and air space control. These GPS systems may operate independently or in combination with inertial reference systems or attitude heading reference systems in order to provide information during an aircraft flight mission.
Global positioning systems similar to NAVSTAR commonly use a satellite information signal receiver system herein referred to generally as a GPS signal receiver system. The GPS signal receiver system is commonly located on a mobile unit for receiving satellite information signals transmitted from a plurality of satellites. Each GPS satellite transmits an information signal containing data that allows a user to determine the range or distance between selected GPS satellites and the antenna associated with the mobile unit's GPS signal receiver system. These distances are then used to compute the position of the GPS signal receiver system, particularly the associated antenna, using known triangulation techniques. For example, in the NAVSTAR-GPS system, a mobile unit, such as an aircraft, with a GPS signal receiver system detects a pseudo random code contained in a given GPS satellite information signal carrier and derives therefrom the "elapsed time" or time delay between the transmission of the signal and its reception at the antenna by the GPS signal receiver system. From this time delay, the GPS signal receiver system can derive the range between the GPS signal receiver system antenna and the satellite, sometimes referred to as the pseudo-range or pseudo-range measurement. Herein, the GPS signal receiver system's position, or the mobile unit's position, generally refers to the corresponding antenna position.
The GPS signal receiver systems known in the art may also determine range by analysis of the satellite information carrier signal. This, of course, is done after the pseudo random code has been captured for subsequent satellite vehicle identification as well as capturing the ephemeris data and/or almanac data from which satellite vehicle position may be determined at selected receiver measurement times. The satellite information signal characteristics, namely phase and frequency, may separately be used for determining range between a specific satellite vehicle and an individual antenna. For example pseudo range rate may be derived from the frequency characteristics and be used to obtain what is referred to as carrier smoothed pseudo range values. Another frequency characteristic is Doppler shift and may be measured by the use of integrated Doppler counts over a fixed time interval as well understood and commonly employed in GPS signal receiver systems known in the art. Lastly carrier phase may be signal process for subsequent determination of antenna position as is also well understood in the art.
As part of the NAVSTAR-GPS system, each satellite information signal also contains precise ephemeris and coarse almanac data which both describe the corresponding satellite's orbital trajectory in earth centered space as is well known in the art. The GPS signal receiver system incorporates signal processing for deriving the either ephemeris data or coarse almanac data from the satellite information signals. In turn, the GPS signal receiver system may derive the geocentric position of the satellite at selected moments of time, generally related to the GPS signal receiver system pseudo range measurement time.
Because of the advancements of GPS technology, the U.S. Federal Aviation Administration is moving toward greater reliance upon satellite positioning systems, and in particular employment of GPS based landing systems, including "differential GPS", thereby eliminating the need for either ILS or MLS landing systems. Accordingly there is a need for enhancing the success of a flight mission intending to rely upon a GPS based landing system.
In particular, users of GPS like systems are particularly concerned about the potential of spoofing. Spoofing is intentionally creating erroneous GPS-like information signals having carrier characteristics being the same as the usual GPS satellite information signals. These spoofing signals may cause the user of such spoofer generated erroneous GPS information signals to be driven off track, particularly during the approach and landing phases of a flight mission.
One method for protecting against GPS signal spoofing is to encrypt the signal transmitted from the satellite vehicles such that no spoofer could emulate a false signal which could spoof a tracked signal. However such a system would be difficult to employ without adding significant costs to the GPS signal receiver system.